Field of the Disclosure
The present disclosure generally relates to a long stroke pumping unit. The long stroke pumping unit may be a linear electromagnetic motor driven long stroke pumping unit or a screw driven direct drive pumping unit.
Description of the Related Art
To obtain hydrocarbon fluids, a wellbore is drilled into the earth to intersect a productive formation. Upon reaching the productive formation, an artificial lift system is often necessary to carry production fluid (e.g., hydrocarbon fluid) from the productive formation to a wellhead located at a surface of the earth. A sucker rod lifting system is a common type of artificial lift system.
The sucker rod lifting system generally includes a surface drive mechanism, a sucker rod string, and a downhole pump. Fluid is brought to the surface of the wellbore by reciprocating pumping action of the drive mechanism attached to the rod string. Reciprocating pumping action moves a traveling valve on the pump, loading it on the down-stroke of the rod string and lifting fluid to the surface on the up-stroke of the rod string. A standing valve is typically located at the bottom of a barrel of the pump which prevents fluid from flowing back into the well formation after the pump barrel is filled and during the down-stroke of the rod string. The rod string provides the mechanical link of the drive mechanism at the surface to the pump downhole.
One such surface drive mechanism is known as a long stroke pumping unit. The long stroke pumping unit includes a rotary motor, a gear box reducer driven by the motor, a chain and carriage linking the reducer to a counterweight assembly, and a belt connecting the counterweight assembly to the rod string. The mechanical drive mechanism is not very responsive to speed changes of the rod string. Gear-driven pumping units possess inertia from previous motion so that it is difficult to stop the units or change the direction of rotation of the units quickly. Therefore, jarring (and resultant breaking/stretching) of the rod string results upon the turnaround unless the speed of the rod string during the up-stroke and down-stroke is greatly decreased at the end of the up-stroke and down-stroke, respectively. Decreasing of the speed of the rod string for such a great distance of the up-stroke and down-stroke decreases the speed of fluid pumping, thus increasing the cost of the well.
Should the sucker rod string fail, there is a potential that the counterweight assembly will free fall and damage various parts of the pumping unit as it crashes under the force of gravity. The sudden acceleration of the counterweight assembly may not be controllable using the existing long stroke pumping unit.